Tilting mask assemblies for fork lift trucks



Nov. 7, 1961 K. s. HANCOCK TILTING MAST ASSEMBLIES FOR FORK LIFT TRUCKS 6 Sheets-Sheet 1 Filed Nov 21, 1958 Nov. 7, 1961 6 Sheets-Sheet 2 \tfl a. 35 7 3 5 9 32 39 I R l A 57 56 2 ZMa/ariw 8? g K. G. HANCOCK 3,007,593.

TILTING MAST ASSEMBLIES FOR FORK LIFT TRUCKS Filed Nov. 21, 1958 Nov. 7, 1961 K. G. HANCOCK 3,007,593

TILTING MAST ASSEMBLIES FOR FORK LIFT TRUCKS Filed Nov. 21, 1958 6 Sheets-Sheet 5 Mew/v0 bum 75 7747;521:001? 75Z/Fr 51'Le'cmf KW W fim X17 341M & 7

Nov. 7, 1961 K. G. HANCOCK TILTING MAST ASSEMBLIES FOR FORK LIFT TRUCKS 6 Sheets-Sheet 4 Filed Nov. 21, 1958 kuwx t Me han t- BER :5 2 9 wQ y QQ-S sm R IIVI Nov. 7, 1961 K. G. HANCOCK 3,007,593

TILTING MAST ASSEMBLIES FOR FORK LIFT TRUCKS 6 Sheets Sheet 5 Filed Nov. 21, 1958 RVN mmm E 5;

vnw mmwK HT lI Nov. 7, 1961 K. G. HANCOCK 3,007,593

TILTING MAST ASSEMBLIES FDR FORK LIFT TRUCKS 6 Sheets-Sheet 6 Filed Nov. 21, 1958 United States Patent Ofihce 3,007,593 Patented Nov. 7, 1961 3,007,593 TILTING MASK ASSEMBLIES FOR FORK LIFT TRUCKS Kenneth George Hancock, Walton, Warrington, England,

assignor to Electro-Hydraulics Limited, Warrington,

England, a company of Great Britain Filed Nov. 21, 1958, Ser. No. 775,542 25 Claims. (Cl. 214-674) The present invention relates to lift trucks (hereinafter referred to as fork lift trucks) having tilting mast assemblies. In certain constructions of fork lift trucks the mast is arranged so that it may be tilted in a forward direction. If the fork or load carrying means is raised and the mast tilted there will be an increase in the turning moment about the front axle of the truck which turning moment is dependent upon the elevation of the fork and load carried thereby, and the angle of inclination of the mast. It will thus be appreciated that if the turning moment exceeds a certain limit dependent upon the weight of the truck and its centre of gravity, the turning moment will cause the truck to tip forward and overturn. An object of the present invention is to provide for limitation of the forward tilt of the mast assembly and the height to which a load may be raised, so as to prevent the possibility of the truck being overturned.

According to the present invention "a fork lift truck includes a tilting mast assembly comprising a mast tilting motor, a load elevating motor, and means responsive to the instantaneous forward tilting moment of the mast assembly, which tilting moment is dependent, at least in part, upon the elevation of the load carrying means and the load carried thereby, and the angle of inclination of the mast, said means being adapted when the tilting moment reaches a predetermined value to render said motor-s incapable of further operation to tilt the mast and elevate the load.

The mast tilting motor and the load elevating motor may each comprise an hydraulic jack.

In one arrangement of the present invention the response means is sensitive to the instantaneous tension 'load in the tilting jack resulting from the instantaneous forward tilting moment of the mast assembly. The response means may comprise a link pivotally connected between one end of the til-ting jack and either the tilting mast or a chassis of the truck, the pivotal axis of at least one of the pivotal connections of the link being offset from the line of action of the tilting jack so that the tension load in said jack would tend to turn said link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached and means adapted to actuate either by movement of the biassing means or by turning movement of the link to cause the tilting jack and elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

In an alternative arrangement of the present invention the response means may be sensitive to the instantaneous load on sprung forward or rear wheels of the truck which lead may vary and results from the instantaneous forward tilting moment of the mast assembly. In this instance the response means may comprise a lever pivoted intermediate its ends to the truck chassis, a link pivotally connected to one end of the lever and the spring suspension means or axle of the forward or rear wheels, whereby movement of the suspension means or axle will cause movement of the lever, and means adapted to be actuated by movement of the lever to cause the tilting jack and elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

In either of the foregoing arrangements the means to cause the jacks to be locked may comprise a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack which valves, upon actuation, close to prevent fluid leaving the annular side of the tilting jack, thereby preventing it from extending further to tilt the mast, and cut-elf the supply of fluid to the full area side of the elevating jack, thereby preventing it from further extension to elevate the load. The valves may be actuated directly by movement of the link, biassing means, or lever as the case may be. Alternatively the valves may be adapted to be actuated indirectly by an hydraulic pump device adapted to be operated by movement of the link, biassing means, or lever, as the case may be.

In a further arrangement of the invention the response means may comprise valve means sensitive to a pressure differential between the full area side and the annular side of the tilting jack, which differential is dependent upon the tension load in the tilting jack, said valve means being automatically opera-ted, when the pressure differential reaches a predetermined limit, hydraulically to lock the tilting jack and elevating jack against further extension.

The valve means may comprise a tilt limiting valve and a lift limiting valve, each in hydraulic circuit with the tilting jack, and independently responsive to the pressure differential in the tilting jack and adapted to close ports respectively in the hydraulic circuits of the tilting jack and the elevating jack. Alternatively the valve means may comprise a tilt limiting valve in hydraulic circuit with the tilting jack and lift limiting valve mechanically coupled to the tilt limiting valve for operation in unison therewith, said valves being adapted to close ports respectively in the hydraulic circuits of the tilting jack and elevating jack. If desired the valve means may comprise a lift limiting valve in hydraulic circuit with the tilting jack and a tilt limiting valve mechanically coupled to the lift limiting valve for operation in unison therewith, as in the previous case, the valves being adapted to close ports respectively in the hydraulic circuits of the elevating jack and tilting jack.

The lift limiting valve and/or tilt limiting valve, in hydraulic circuit with the tilting jack, may comprise a piston slidable in a cylinder, said piston having a full area side and an annular side respectively in hydraulic communication with the full area side and annular side of the tilting jack, whereby pressure differential in the tilting jack in excess of the predetermined limit will cause movement of the piston to close a port or ports in the hydraulic circuits of the tilting jack and elevating jack.

The means responsive to the overturning moment may be adapted to actuate a visual and/or audible warning means when the overturning moment of the tilting mast assembly reaches the predetermined value. The invention is illustrated diagrammatically in the accompanying drawings in which:

FIGURE 1 is a side elevation of part of a fork lift truck, with its mast assembly in a vertical position;

FIGURE 2 is a side elevation of the fork lift truck with its mast assembly tilted forward;

'FIGURE 3 shows one embodiment of a hydraulic circuit adapted to control operation of the tilting mast assembly;

FIGURE 4 shows an hydraulic circuit including one form of means responsive to a tension load in a tilting jack;

FIGURE 5 shows an hydraulic circuit including an alternative arrangement of means responsive to a tension load;

FIGURE 6 shows an hydraulic circuit including one form of means responsive to a pressure differential in a tilting jack;

FIGURE 7 is shows an hydraulic circuit including one form of means responsive to the load on the forward or rear wheels of a truck, and

FIGURE 8 shows an alternative arrangement of the load responsive means.

Referring to FIGURES 1 and 2, the fork lift truck has a mast E pivoted at the point A to the truck and a load carrying platform or fork 'F slidable up and down the mast by an elevating jack (not shown) in known manner. The mast is tilted by means of an hydraulic jack J pivotally coupled to the truck at B and to the mast at C. As will be seen in FIGURE 2 of this drawing the raising and lowering of a load and/ or tilting of the mast E not only varies the turning moment about the front axle but also varies the pressure of the fluid on the annular side of the tilting jack J relative to the pressure on the full area side thereof, due to a tension load in said jack caused by the turning moment.

In FIGURE 3 there is illustrated a hydraulic circuit including means responsive to the tension load in the tilting jack and adapted to control operation of a tilting mast assembly of a fork lift truck so that overturning of the truck is prevented.

A fluid feed line '1 from a pump (not shown) leads to a tilt selector 2 and a lift selector 3. From the tilt selector 2 there runs a feed line 4 to the full area side 5 of a tilting jack 6. Likewise, from the lift selector 3 there runs a feed line 7 through a lift limiting valve 8 via ports 9 and annular passage 10, to the full area side 11 of an elevating jack 12. A piston 13 of the tilting jack 6 is coupled through suitable linkage (not shown) to a tilting mast assembly (not shown). The annular side 14 of the tilting jack communicates through a feed line 15 with the lift limiting valve 8 and a tilt limiting valve 16, and through a second feed line 17 with the tilt limiting valve 16 only.

The tilt limiting valve -16 includes a piston 18 spring biased on its full area side 19 by a coil spring 20. A spool 21 formed with an annular peripheral chamber 22, is slidable within the valve 16 and the chamber 22, when the spool 21 is in a normal position as shown in the accompanying drawing, communicates with an inlet port 23, coupled to the feed line 17, and an outlet port 24. A fluid outlet line 26 is coupled to the outlet port 24 and leads from the tilt limiting valve 16 back to the tilt selector 2 from which there runs an outlet conduit 27 to a fluid supply tank or reservoir tank (not shown) of the hydraulic circuit. The feed line 15 from the tilt jack 6, is in communication with a pressure chamber 27 in the tilt limiting valve 16. A bypass passage 28 is coupled to the feed line 15 and fluid outlet line 26 and incorporates a non-return valve 29 through which fluid may pass only from the outlet line 26 to feed line 15.

The full area side 19 of the tilt limiting valve 16 communicates with a full area side 30 of the lift limiting valve 8 through a fluid line 31, which line is in open connection with the feed line 4 from the tilt selector 2. The lift limiting valve 8 includes a piston 32 which is spring biased on its full area side 30 by a coil spring 33. Slidably mounted within the valve is a spool 34 having a peripheral groove 35 defining the annular passage 10. The feed line 15 from the tilting jack 6 leads to a pressure chamber 36 in the lift limiting valve. Coupled to the feed line 7 of the lifting jack and by-passing the lift limiting valve 8, is a fluid-by-pass line 37 incorporating a pressure release valve 38. A pressure differential gauge 39 is coupled to the lines 15 and 31 so as to give visual indication of the pressure differential acting on the full area sides 19 and 30 of the tilt and lift limiting valves 16 and 8 and in the pressure chambers 27 and 36.

It will be appreciated that the pressures acting on the full area sides of the tilting jack 6, tilt limiting valve 16 and lift limiting valve '8 will be the same at all times, and likewise those on the annular side 14 and chambers 27 and 36 correspond. If the tilting jack is operated to tilt the mast assembly, that is to say fluid is pumped into said jack on the full area side so that it is extended, turning moments created by such tilt or inclination will set up a tension load in the jack and hence an increase in hydraulic pressure on the annular side of the jack with respect to that on the full area side. The annular side pressure in the tilting jack is a function of the full area pressure and tension load and having regard to the fact that the full areas and annular areas of the tilting jack v-alve, tilt limiting valve, and lift limiting valve respectively correspond, the tilt limiting valve and lift limiting valve will be sensitive to such increase in hydraulic pressure.

Considering the tilt limiting valve 16, an increase in hydraulic pressure in the annular side 14 of the tilting jack 6 will act, through the feed line 15, in the chamber 27 of said tilt limiting valve. An increase in pressure in the chmber 27 will tend to move the spool 21 to the right and thereby move the annular chamber 22 out of register with the ports 23 and 24. When this occurs escape of fluid from the annular side 14 of the tilt jack 6 is prevented thereby locking said jack against further extension. With the spool 21 in this locking position closing the ports 23 and 24, the tilt jack is released by turning the tilt selector 2 to connect line 1 to line 26 and opening the valve 29 and pumping fluid through the lines 26, 28 and 15 to the annular side 14 of said jack. ,If the ports 23 and 24 have not been closed by the spool 21, then fluid may be pumped into the annular side 14 of the jack 6 through the annular chamber 22 and line 17.

Any pressure increase in the annular side 14 of the tilting jack 6 also acts through the feed line 15 in the chamber 36 of the lifting limiting valve 8. This increase of pressure in the chamber 36 tends to move the spool 34 to the right so as to move the annular passage 10 in said spool out of register with the ports 19 and when this occurs flow of fluid through the feed line 7 to the lifting jack 12 is shut off. To release the elevating jack when the lift limiting valve is in a position hydraulically to lock further elevation thereof, the pressure release valve 38 is actuated, thereby allowing outflow of fluid from the full area side 11 in said jack through the by-pass 37 and lift selector 3.

It will thus be seen that both the tilting jack and elevating jack are hydraulically locked against further ti'ting or lifting movement. By suitably dimensioning the effective full area sides and annular sides of the tilting and elevating jacks and tilt and lift limiting valves, the tilt and lift limiting valves may be made responsive to predetermined pressure differences in the tilting jack depending upon the characteristics of the tilting mast assembly and What limitations thereof are required.

The visual pressure indicating gauge 39 is provided so as to give a visual warning to the operator when the limiting position of the mast assembly is being reached.

In FIGURE 4 of the accompanying drawings there is illustrated diagrammatically an alternative arrangement of a tilting mast assembly. A tilting mast is pivoted at 101 to a fork lift truck (not shown). The forks of the mast are raised and lowered by an elevating jack 102 and the mast is tilted by a tilting jack 103. The tilting jack 103 is pivotally coupled at one end at 104 to the fork lift truck and at its other end is pivotally connected at 105 to a link 106. This link is pivoted about an axis 107 to a bracket 108 carried by the mast 100. The pivotal axis 107 is offset from the line of action of the tilting jack 103, which line of action is indicated by an arrow 109. An arm 110 extends downwardly from the link 106 and is attached at its lower end to one end of a tension spring 111, which at its other end is secured to a bracket 112 carried by the mast 100. The tension spring 111 biasses the link 106 in an anti-clockwise direction about the axis 107. An abutment finger 113 carried by the arm 110 is arranged to actuate, as will be described hereinafter, a micro-switch 114- and the arm 110* is arranged, as will be described hereinafter to move a plunger 115 of a hydraulic pump device 116.

The tilting jack 103' is supplied with hydraulic fluid from a tilt selector (not shown) through feed lines 117 and 118. Feed line 117 leads to the annular side of the jack 103 and the feed line 118 leads to the full area side. A tilt limiting valve 119 is disposed in the line 117 and it comprises a cylinder 120 in which is slidable a spool 121 formed with an annular passage 122. In its normal position of operation the spool 121 lies within the cylinder 120 so that the annular passage 122 is in communication with ports 123 in the Walls of the cylinder 120 and connected with the feed line 117. A pressure chamber 124 is formed in one end of the cylinder 120 and is supplied with pressure fluid through a line 125 from the hydraulic pump 116. It thus follows that a pressure increase in the chamber 124 will tend to move the spool 121 towards the right as shown in the accompanying drawing. This movement is resisted by a compression spring 126 until the pressure exceeds a predetermined value. A fluid line 127 by-passes the tilt limiting valve 119 and includes a non-return valve 128. This valve can be operated only to allow pressure fluid to'be supplied to the annular side of the tilt jack 103, should the spool 121 be in a position closing either of the ports 123- in the tilt limiting valve 119.

The elevating jack 102 is supplied with fluid pressure through a line 130 from a selector (not shown). A lift limiting valve 131 is disposed in this line 130 which lift limiting valve is similar in construction and operation to the tilt limiting valve 119, like reference numerals being used to indicate like parts. A feed line 132 connects the pressure chamber 124 of the lift limiting valve 131 to the hydraulic pump 116 and non-return valve 133 is located in a feed line 134 which by-passes the lift limiting valve 131.

When it is desired either to elevate the fork of the mast assembly or to tilt the mast, the selectors of the elevating jack 102 and tilting jack 103 are set accordingly and fluid pressure supplied to the full area sides of said jacks which operate in known manner to elevate the forks and tilt the mast. As the mast is tilted so the turning moment, due to the weight and height of the load being carried by the fork, and the angle of inclination of the mast, will cause a tension load to act in the tilting jack 103. This will result in a pressure increase in the annular side of the jack with respect to its full area side. Pressure in the annular area reacts the sum of the tension load and full area pressure. For a given full area pressure, an increase in tension load results in an increase of annular area pressure. If this tilting moment is less than the moment suflicient to cause overturning of the truck then the tilting jack and elevating jack will continue to operate in known manner. If, however, the tilting moment approaches the overturning moment then the tension load in the tilting jack 103 will overcome the action of the biassing tension spring 111 and hence cause the link 106 to pivot about the axis 107 tending to cause the pivotal connections of the link 106 to the tilting jack 103 and bracket 108, to align. As the link 106 turns the abutment finger 113, carried by the arm 110, actuates the micro-switch 114 which closes an electrical circuit for a warning lamp indicated at 140 and also if desired, an audible warning device, for example a buzzer or a bell. The arm 110, as the link 106 turns, is brought to bear against the plunger 115 of the hydraulic pump device 116. This causes pressure fluid to be forced from the hydraulic pump cylinder 116, through lines 125 and 132, into the pressure chambers 124 of the tilt limiting valve 119 and the lift limiting valve 131. When the pressure in these two chambers reaches a certain value the action of the compression springs 126 is overcome so that the spools 121 of each valve may move to the right, as shown in the accompanying drawings, so that the annu- 6 lar passages 122 in the spools 121 are not in communication with the ports 123, thereby cutting oif a supply of pressure fluid to the full area side of the elevating jack 102 and preventing pressure fluid from leaving the annular side of the tilting jack 103. The elevating jack and tilting jack are thus hydraulically locked against further extension so that overturning of the truck is prevented.

The elevating jack and tilting jack cannot now be actuated respectively to raise the forks or tilt the masts further, but the non-return valves 133 and 128 permit pressure fluid to be withdrawn from the full area side of the elevating jack and supplied to the annular side of the tilting jack so that the fork may be lowered or the mast drawn back towards its vertical or normal position.

If the fork is lowered then the turning moment will decrease resulting in a decrease in the tension load in the tilting jack. This will allow the tension spring 111 to turn the link 106 back into its normal position so that the arm 110 no longer 'bears against the plunger and the micro-switch 114 is released. When the plunger 115 of the hydraulic pump device 116 is released, so the fluid in the pressure chambers 124 of the tilt limting valves 119 and 131 is allowed to flow back into said pump and the spools 121 of these valves to return to their normal positions under the action of the springs 126. The annular passages 122 of the spools 121 are thus brought back into register with the ports 123. It will thus be seen that if the fork is lowered this brings about a resulting decrease in the turning moment and then the mast can be inclined forwardly to a greater extent without overturning of the truck occurring. Alternatively if the mast is returned towards its vertical position the turning moment will decrease and the fork can be raised to a further extent.

If in elevating a light load the overturning movement of the truck is not reached then the tilting and lifting jacks operate in the usual manner, the jacks coming up against their internal stops when they are either fully extended or fully retracted.

It will of course be appreciated that the tension spring 111, which acts as a biassing means to counteract movement of the link 106 until a predetermined tension load in the tilting jack has been exceeded, has characteristics depending upon the weight of the fork lift truck, and its centre of gravity; the greater the weight of the truck and the further aft its centre of gravity the greater the turning moment will have to be to cause the truck to overturn, and therefore a heavy tension spring is used. With this arrangement a load may be lifted by the fork and the mast tilted without risk of overturning the truck because, before the overturning moment is reached, the tilting jack and elevating jack are hydraulically locked against further extension until some action is taken by an operator to decrease the turning moment.

If desired, the hydraulic pump device 116 may be replaced by an electrical switch and the limiting valves 119 and 131 may be solenoid operated. In this arrangement when the link 106 turns, the arm 110 would actuate this electrical switch and cause it to close electrical circuits of the solenoids whereby they would move the spools 121 of the valves 119 and 131 in the manner described hereinbefore. Alternatively the movement of the biassing spring may be used directly to operate the tilt and lift limiting valves.

The tilt limiting valve 119 and the lift limiting valve 131 may be actuated directly by the arm 110 of the link 106 and such an arrangement is illustrated in FIGURE 5. The spool 121 of each valve is provided with a plunger protruding from the valve into the line of movement of the arm 110, so that, when the arm is moved as hereinbefore described, it bears against the plungers 150 moving them to the left as shown in the accompanying drawings, whereby the ports 123 of the valves 119 and 131 are closed by the spools 121. Flow of hydraulic fluid from the annular side of the tilting jack,

7 and to the full area side of the elevating jack, is thus prevented and the jacks lock against further extension.

In FIGURE 6 of the accompanying drawings there is shown an alternative hydraulic circuit adapted to control operation of a tilting mast assembly to a fork lift truck. A fluid feed line 200 from a pump (not shown) leads to a tilt selector 201 and a lift selector 202. From the tilt selector there runs a feed line 203 to the full area side 204- of a tilting jack 205. Likewise from the lift selector 202 there runs a feed line 206 through a lift limiting valve 207 to the full area side 208 of an elevating jack 209. A piston 210 of the tilting jack 205 is coupled to a tilting mast (not shown). An annular side 211 of the tilting jack 205 communicates through a feed line 212 with the tilt selector 201 and a tilt limiting valve 215 is disposed in this line. A second feed line 227 runs from the annular side 211 of the tilting jack 205 and is coupled to the feed line 212 between the tilt limiting valve 215 and tilt selector 201. The tilt limiting valve 215 comprises a cylinder 216 in which there is slidable a piston 217 formed with an annular passageway 218. This annular passageway 218 communicates with ports 219 in the walls of the cylinder 216 whereby fluid flowing through the feed line 212 may pass through the ports 219 into the passageway 218 and hence pass through this valve, The pressure on the full area side 220 of the piston 217 acts in a pressure chamber 221 supplied with fluid under pressure by a feed line 222 which is coupled to the feed line 203 so that the pressure acting in this pressure chamber is the same as that acting on the full area side of the tilting jack 205. Also acting on the full area side 220 is a compression spring 223 biassing the piston 217 towards the right as shown in the accompanying drawings. An annular side 225 of the piston 217 acts in a chamber 226 which is supplied by fluid under pressure through a feed line 238 coupled to the feed line 227. It will thus be appreciated that the pressure in the chamber 221 and acting on the full area side 220 of the tilt limiting valve 215 corresponds to the pressure acting on the full area side 204 of the tilting jack, and that the pressure acting on the annular side 225 of the tilt limiting valve 215 likewise corresponds to that acting on the annular side 211 of the tilting jack 205; the ratio of the areas, full area side; annular side of the tilt limiting valve and tilting jack, are equal. A pressure release valve 230 is coupled into the feed line 227.

This valve 230 comprises a valve member 231 which seats on a valve seat 232 separating a first chamber 233 from a second chamber 234. The valve member 231 may be lifted from its seat 232 by a spring biassed plunger 235 which is moved into the valve 230, against its biassing spring, to lift the valve member 231 off its seat by an abutment finger 236 carried by a piston rod 240 of the tilting jack 205. The abutment finger 236 moves with the piston 210 of the tilting jack 205 so that when the jack is in its fully retracted position, with the tilting mast assembly vertical or rearwardly inclined, the valve 230 is opened. When the tilting jack is initially extended the plunger 235 moves out of the valve 230 under influence of its biassing spring so allowing the valve member 231 to move on to its valve seat 232 thereby closing the valve 230.

The lift limiting valve 207 is mechanically coupled to the tilt limiting valve 215. This lift limiting valve comprises a spool 241 slidable in a cylinder 242. The spool is coupled through a linkage 243 to the piston 217 of the tilt limiting valve 215 so that said piston and spool move in unison. The spool 241 is formed with an annular passage 244 which in its normal position communicates with ports 245 coupled to the feed line 206 so that fluid may flow through said line and the annular passage 244 to the elevating jack 209, A non-return valve 246 is coupled to a bypass feed line 247 by-passing the lift limiting valve 207. This non-return valve 246 permits 8 pressure in the elevating jack 209 to be released as will be described hereinafter.

As hereinbefore mentioned a tension load in the tilting jack causes a pressure increase in the annular side of the jack with respect to the full area side thereof. With the arrangement described above and illustrated in FIGURE 6 a pressure increase in the annular side 211 of the tilting jack 205 will cause a corresponding pressure increase in the annular side 225 of the tilt limiting valve 215. Although there is this pressure increase on the annular side of the valve the piston 217 is prevented from moving toward the left, as shown in the drawing, by the biassing spring 223 until such time as the pressure differential between the annular side and full area side of the tilting jack, which differential corresponds to that between the annular side and full area side of the til limiting valve, exceeds a predetermined value whereupon the biassing action of this spring 223 is overcome and the piston 217 is caused to move to the left. Movement of the piston to the left carries the annular passage 213 out of register with the ports 219 so that said ports are closed thus preventing fluid from leaving the annular side of the tilting jack so that said jack cannot be further extended to tilt the mast to a greater angle. Fluid cannot leave the annular side 211 of the tilting jack 205 through the feed line 227 because the valve 230 is closed.

As the piston 217 of the lift limiting valve 215 moves it carries with it the spool 241 of the lift limiting valve 207 due to the linkage 243. This causes the annular passage 244 in the spool 241 to move out of register with the ports 245 so that they are shut and supply of fluid through the feed line 206 to the elevating jack 209 is prevented.

It will thus be seen that, when the pressure differential between the annular side and full area side of the tilting jack exceeds a predetermined value, the outlet of hydraulic fluid from the annular side of the tilting jack is closed and supply of fluid to the elevating jack is shut off so that both of said jacks cannot be further extended. This pressure differential results from the tension load in the tilting jack and by suitably choosing a spring 223 to bias the piston 217 so said piston can be prevented from movement until the chosen value of pressure differential is exceeded. The tension load is directly related to the overturning moment of a fork lift truck so that, depending upon the size and weight of the fork lift truck, a spring 223 having the desired characteristics can be chosen.

If the piston 217 and spool 241 have been moved to close their respective ports 219 and 245, the jacks 205 and 209 cannot be further extended. They can however be retracted and in the case of the elevating jack 209 this is achieved by turning the lift selector 202 to the down position. With the lift selector in this position fluid can flow from the jack through the line 247 and non-return valve 246 and hence back to the storage tank of the hydraulic system.

As the elevating jack is retracted so a load carried by the fork and the tilting mast assembly will be lowered causing a decrease in the value of the turning moment. This will result in a decrease in the tension load acting in the tilting jack which in turn will cause a drop in the pressure differential between the annular side and full area side of the tilting jack namely a lowering of pressure in the annular side. This pressure drop in the annular side 211 of the tilting jack 205 is also recorded in the annular side 225 of the tilt limiting valve 215 and thus the pressure differentials in the tilting jack and limiting valve are allowed to reduce and the piston 217 will move back to its normal position under the influence of the biasing spring 223 with the annular passage 218 in register with the ports 219. Simultaneously the spool 241 of the lift limiting valve 207 is moved back into its normal position with the annular passage 244 in register with the ports 245. Thus the system is returned to normal so that the elevating jack 209 and tilting jack 205 may be extended and retracted as desired, provided of course the predetermined pressure differential in the tilting jack does not exceed the predetermined value.

If the tilt limiting valve 215 has been actuated to prevent further extension of the tilting jack, this jack can be retracted to return the tilting mast assembly to its vertical or rearwardly inclined position simply by turning the tilt selector to its vertical position. In this position the tilt selector directs fluid under pressure from the line 200 through the line 227 and valve 230 into the annular side 211 of the tilting jack 205 and the annular side 225 of the tilt limiting valve 215. Since the valve 230 is a one way valve, fluid in the chamber 233, at a pressure greater than that in the chamber 234, can flow from the chamber 233 into the chamber 234. As the tilting jack 205 is retracted, so the pressure differential between the full area side and annular side of said jack and also of the tilt limiting valve 215 will decrease and when this falls below the predetermined value, which corresponds to the overturning moment of the truck, the piston 217 of the tilt limiting valve 215 can return to its normal position under influence of the biasing spring 223. When the tilting jack 205 is fully retracted, the abutment finger 236 bears against the plunger 235 to hold the valve member 231 off its valve seat 232. With the valve 230 open, the tilt limiting valve 215 is in effect rendered inoperative so that when fluid pressure is introduced into the chamber 204 to act on the full area side of the piston 210 of the tilting jack 205, fiuid can escape from the annular side of the jack not only through the line 2 12 but also through the line 227. After the jack has extended a predetermined amount, the valve member 231 is moved on to its seat 232 thereby closing the valve 230, but fluid can still escape from the annular side of the tilting jack through the line 212 until such time as the overturning moment of the truck is reached, when the valve 215 operates as described hereinbefore.

Fluid lines may be taken from the annular side and full area side of the tilting jack to a visual and/or audible warning device, which lines are indicated at 250. The visual warning device could conveniently take the form of a simple pressure gauge showing the pressure difference between the annular side and full area side of the tilting jack, the gauge being marked to show when the pressure differential is reaching the predetermined value which corresponds to the overturning moment of the truck.

In FIGURES 7 and 8 there are illustrated hydraulic circuits including means responsive to the load on the forward or rear wheels of the truck. As the mast is tilted forward and the fork, and load carried thereby, elevated, there will be an increase in the turning moment about the front axle of the truck, resulting in a transfer of load from the rear wheels to the forward wheels. This redistribution of Weight between the forward wheels and rear wheels results in the spring suspension means of the forward wheels being compressed and those of the rear wheels being released. According to a further embodiment of the present invention this movement of the suspension means is used to actuate the response means of the tilting jack and elevating jack. The hydraulic circuit in this embodiment is the same as that described hereinbefore with reference to FIGURES 4 and 5. This embodiment differs from that previously described in the manner in which the hydraulic pump device 116 is operated.

A rear wheel 300 of the truck is rotatably mounted on an axle 301 which is secured by U-bolts 302 to a composite leaf spring 303 pivotally connected at its ends to the chassis of the truck as indicated at 304. A link 305 is pivotally connected at its lower end to the axle 301 and at its upper end to one end of a lever 306. This lever 306 is pivotally connected, about an axis 307 intermediate its end, to a bracket 308 carried by the chassis of the truck. The other end 309 of the lever 306 is formed 10 with an abutment face 310 adapted to bear against the plunger of the hydraulic pump device 116.

Having regard to the foregoing description it will be appreciated that if the lever 306 pivots about the axis 307 so that the abutment face 310 is brought to bear against the plunger 115 so that it is moved into the pump 116, the ports 123 in the tilt limiting valve 119 and lift limiting valve 131 will be closed.

Whena load is elevated by the forks and the mast assembly is tilted there is a weight transfer from the rear wheels to the forward wheels so that the rear end of the chassis tends to lift. When this occurs there will be movement of the lever 306 about the axis 307 because the axle 30-1 remains substantially stationary. The abutments face 310 of the lever 306 will thus be brought to bear against the plunger 115 of the hydraulic pump device 116. By suitably choosing a leaf spring 303 so that it is the desired spring rate dependent upon the Weight and centre of gravity of the truck, the lever 306 can be arranged to actuate the pump device 116 just before the overturning moment of the truck is reached. When the pump device 116 is actuated the hydraulic circuit performs in the manner hereinbefore described with reference to FIGURE 4.

The means responsive to the load on the Wheels of a fork lift truck may be adapted to be actuated by the load on either the forward wheels or rear wheels. The foregoing description relates to the application to the rear wheels but by, for example, dispensing with the lever 306 and arranging for the link 305 to act directly on the plunger 115, so a load transferred to the forward wheels of the truck may bring the hydraulic pump device 116 into operation.

Furthermore the hydraulic pump device 116 may be dispensed with and the tilt limiting valve 119 and lift limiting valve 131 may be actuated directly by the load sensitive means. In this case the spools 121 of the tilt and lift limiting valves are formed with plungers 350 (FIGURE 8) extending from said valves into the path of movement of the abutment face 310 of the lever 306 or of the link 305.

What I claim is:

l. A fork lift truck including a tilting mast assembly comprising a mast tilting motor, a load elevating motor, and means responsive to the instantaneous forward tilting moment of the mast assembly, which tilting moment is dependent, upon the elevation of the load, the weight of the load, and the angle of inclination of the mast, said means being adapted, when the tilting moment reaches a predetermined value, to render both said load elevating and mast tilting motors incapable of further operation to tilt the mast and elevate the load.

2. A fork lift truck according to claim 1, wherein the mast tilting motor and the load elevating motor each comprises a hydraulic jack.

3. A fork lift truck according to claim 2, wherein the response means is sensitive to the instantaneous tension load, in the tilting jack, resulting from the instantaneous forward tilting moment of the mast assembly.

4. A fork lift truck according to claim 3, wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to the tilting mast, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached and means, adapted to be actuated by turning movement of the link, to cause the tilting jack and elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

5. A fork lift truck according to claim 2, wherein the response means is sensitive to the instantaneous load on sprung Wheels of the truck, which load may vary and re sult from the instantaneous forward tilting moment of the mast assembly.

6. A fork lift truck according to claim 5, wherein the response means comprises a lever pivoted intermediate its ends to the truck chassis, a link pivotally connected between one end of the lever and the spring suspension means, of the truck wheels, whereby movement of the suspension means will cause movement of the lever, and means adapted to be actuated by movement of lever, to cause the tilting jack and elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

7. A fork lift truck according to claim 2, wherein the response means comprises valve means sensitive to a pressure differential between the full area side and the annular side of the tilting jack, which differential is dependent upon the tension load in the tilting jack, said valve means being automatically operated when the pres sure differential reaches a predetermined limit hydraulically to lock the tilting jack and elevating jack against further extension.

8. A fork lift truck according to claim 7, wherein the valve means comprises a tilt limiting valve and a lift limiting valve, each in hydraulic circuit with the tilting jack, and independently responsive to pressure differential in the tilting jack and adapted to close ports respectively in the hydraulic circuits of the tilting jack and elevating jack.

9. A fork lift truck according to claim 7, wherein the valve means comprises a tilt limiting valve in hydraulic circuit with the tilting jack and a lift limiting valve, mechanically coupled to the tilt limiting valve for operation in unison therewith, said valves being adapted to close ports respectively in the hydraulic circuits of the tilting jack and elevating jack.

10. A fork lift truck according to claim 7, wherein the valve means comprises a lift limiting valve in hydraulic circuit with the tilting jack, and a tilt limiting valve mechanically coupled to the lift limiting valve for operation in unison therewith, the valves being adapted to close ports respectively in the hydraulic circuits of the elevating jack and tilting jack.

11. A fork lift truck according to claim 8 wherein the tilt limiting valve and the lift limiting valve each comprises a piston slidable in a cylinder, said piston having a full area side and an annular side respectively in bydraulic communication with the full area side and annular side of the tilting jack, whereby pressure differential in the tilting jack in excess of the predetermined limit will cause movement of the piston to close a port in the hydraulic circuits of the tilting jack and elevating jack.

12. A fork lift truck according to claim 3, wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to the tilting mast, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached, and valve means, adapted to be actuated by turning movement of the link, comprising a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack, which valves, upon actuation, close to prevent fluid leaving an annular side of the tilting jack, thereby preventing it from extending further to tilt the mast, and to cut off the supply of fluid to a full area side of the elevating jack, thereby preventing it from further extension to elevate the load.

13. A fork lift truck according to claim 3 wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to the tilting mast, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached, and means, adapted to be actuated by movement of the biassing means, to cause the tilting jack and the elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

14. A fork lift truck according to claim 3 wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to the tilting mast, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached, and valve means, adapted to be actuated by movement of the biassing means, comprising a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack, which valves, upon actuation, close to prevent fluid leaving an annular side of the tilting jack, thereby preventing it from extending further to tilt the mast, and to cut off the supply of fluid to a full area side of the elevating jack, thereby preventing it from further extension to elevate the load.

15. A fork lift truck according to claim 3 wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to a chassis of the truck, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means to prevent such alignment until the predetermined tension load is reached, and means adapted to be actuated by turning movement of the link to cause the tilting jack and the elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

16. A fork lift truck according to claim 3 wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to a chassis of the truck, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means to prevent such alignment until the predetermined tension load is reached, and valve means, adapted to be actuated by turning movement of the link, comprising a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack which valves, upon actuation, close to prevent fluid leaving an annular side of the tilting jack, thereby preventing it from extending further to tilt the mast and to cut off the supply of fluid to a full area side of the elevating jack, thereby preventing it from further extension to elevate the load.

17. A fork lift truck according to claim 3 wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to a chassis of the truck, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached, and means adapted to be actuated by movement of the biassing means to cause the tilting jack and the elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

18. A fork lift truck according to claim 3 wherein the response means comprises a link pivotally connected to one end of the tilting jack and pivotally connected to a chassis of the truck, the pivotal axis of at least one of the pivotal connections being ofiset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached, and valve means, adapted to be actuated by movement of the biasing means, comprising a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack, which valves, upon actuation, close to prevent fluid leaving an annular side of the tilting jack, thereby preventing it from extending further to tilt the mast, and to cut ofl. the supply of fluid to a full area side of the elevating jack, thereby preventing it from further extension to elevate the load.

19. A fork lift truck according to claim 2 wherein the response means is sensitive to the instantaneous load on the sprung rear wheels of the truck, which load may vary and result from the instantaneous forward tilting moment of the mast assembly.

20. A fork lift truck according to claim 5, wherein the response means comprises a lever pivoted intermediate its ends to the truck chassis, a link pivotally connected between one end of the lever and the spring suspension means of the forward wheels, whereby movement of the suspension means will cause movement of the lever, and valve means, adapted to be actuated by movement of the levers when the instantaneous load on the sprung forward wheels reaches a predetermined value, said valve means comprising a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack, which valves, upon actuation, close to prevent fluid leaving an annular side of the tilting jack, thereby preventing it from extending further to tilt the mast and to cut olf the supply of fluid to a full area side of the ele vating jack, thereby preventing it from further extension to elevate the load.

21. A fork lift truck according to claim 19 wherein the response means comprises a lever pivoted intermediate its ends to the truck chassis, a link pivotally connected between one end of the lever and the spring suspension means of the rear wheels, whereby movement of the suspension means will cause movement of the lever, and means adapted to be actuated by movement of the lever to cause the tilting jack and elevating jack to be locked against operation to tilt the mast and elevate the load respectively.

22. A fork lift truck according to claim 19 wherein the response means comprises a lever pivoted intermediate its ends to the truck chassis, a link pivotally connected between one end of the lever and the spring suspension means of the rear wheels, whereby movement of the suspension means will cause movement of the lever, and valve means adapted to be actuated by movement of the lever when the instantaneous load on the sprung rear wheels reaches a predetermined value, said valve means comprising a valve in the hydraulic circuit of the tilting jack and a valve in the hydraulic circuit of the elevating jack, which valves, upon actuation, close to prevent fluid leaving an annular side of the tilting jack, thereby preventing it from extending further to tilt the mast and to cut off the supply of fluid to a full area side of the elevating jack, thereby preventing it from further extension to elevate the load.

23. A fork lift truck according to claim 9 wherein the tilt limiting valve comprises a piston slidable in a cylinder, said piston having a full area side and an annular side respectively in hydraulic communication with the full area side and annular side of the tilting jack, whereby pressure differential in the tilting jack in excess of the predetermined limit will cause movement of the piston to close ports in the hydraulic circuits of the tilting jack and elevating jack.

24. A fork lift truck according to claim 10 wherein the lift limiting valve comprises a piston slidable in a cylinder, said piston having a full area side and an annular side respectively in hydraulic communication with the full area side and the annular side of the tilting jack, whereby pressure differential in the tilting jack in excess of the predetermined limit will cause movement of the piston to close ports in the hydraulic circuits of the tilting jack and the elevating jack. 1

25. A fork lift truck including a tilting mast assembly comprising a mast tilting motor, a load elevating mo tor, said motors comprising hydraulic jacks, and means responsive to the instantaneous forward tilting moment of the mast assembly, which tilting moment is dependent, at least in part, upon the elevation of the load carrying means and the load carried thereby, and the angle of inclination of the mast, said means being adapted, when the tilting moment reaches a predetermined value, to render said motors incapable of further operation to tilt the mast and elevate the load, said response means comprising a link pivotally connected to one end of the tilting jack and pivotally connected to the tilting mast, the pivotal axis of at least one of the pivotal connections being offset from the line of action of the tilting jack, so that the tension load in said jack will tend to turn the link and align said offset pivotal connection with the line of action of the jack, biassing means adapted to prevent such alignment until the predetermined tension load is reached, a valve in the hydraulic circuit of the tilting jack, a valve in the hydraulic circuit of the elevating jack, which valves, upon actuation, close to prevent fluid leaving the annular side of the tilting jack, thereby preventing it frmo extending further to tilt the mast and to cut off the supply of fluid to the full area side of the elevating jack, thereby preventing it from further extension to elevate the load, and a hydraulic pump device adapted to be operated by turning movement of the link when the predetermined tension load is reached to actuate said valves.

References Cited in the file of this patent UNITED STATES PATENTS 2,659,505 Shaffer Nov. 17, 1953 2,759,563 Marnon et al. Aug. 21, 1956 2,767,394 I Arnot et al. Oct. 16, 1956 2,937,777 IRoot May 24, 1960 FOREIGN PATENTS 947,449 Germany Aug. 16, 1956 

